diff --git a/WeighAndGrading/GradeFrom.cs b/WeighAndGrading/GradeFrom.cs
index ab4314b..b9491d0 100644
--- a/WeighAndGrading/GradeFrom.cs
+++ b/WeighAndGrading/GradeFrom.cs
@@ -172,6 +172,7 @@ namespace WeighAndGrading
           row.DefaultCellStyle.BackColor = Color.YellowGreen;
       }
       dataGridView.ClearSelection();
+      tangRow = maoRow = null;
     }
 
     void BindDetailGrid()